Switching device in a direct current circuit for transferring a current from one current path to another current path

Abstract: Device in a direct current circuit for transfer of a direct current (I d1 ) between a first current path (e.g. SG-X1-P41-SR1) and a second current path (GE-EL-PO-NS-Z1-Y1) and/or for limiting the voltage at a point (P51) in the circuit. The device comprises a first semiconductor connection (X1), which is connected only to the first current path and is controllable between conducting and non-conducting states and vice versa, a second semiconductor connection (Y1), which is connected only to the second current path and has a diode function, a branch point (P41) at which the two current paths are connected to each other, a capacitor (C11) connected between those ends of the semiconductor connections which are facing away from the branch point, and control members adapted to control the first semiconductor connection between conducting and non-conducting states and vice versa in dependence on the voltage (U C11 ) across the capacitor.

Description: The invention relates to switching device according to the precharacterising part of claim 1. The invention relates especially to switching devices in circuits for high direct currents and/or high direct voltages, such as plants for power transmission by means of high-voltage direct current (HVDC plants/HVDC transmissions). In a plant for HVDC power transmission switchings of the direct current in a converter station between different current paths are made in certain operating cases. In a monopolar HVDC transmission, or during monopolar operation of a bipolar transmission, the operating current of a converter station flows in undisturbed operation via a ground electrode. Normally, this ground electrode is located at a distance from the station and is connected thereto through an electrode line, often of considerable length. An HVDC station normally has a neutral busbar, by which is meant in this application a busbar in an HVDC station which is intended, during normal operation, to lie at a potential near ground. Such a busbar is connected to or connectible to the ground electrode via the electrode line. Neutral busbars normally exist in the converter stations both in monopolar and in bipolar HVDC transmissions. In connection with certain faults, for example an interruption in the electrode line, it is known to temporarily connect the neutral busbar to a station ground with the aid of a mechanical circuit breaker, the station ground being a ground electrode arranged in the station or near the station. After termination of the disturbance, the circuit breaker has.been opened, whereby the operating direct current has been transferred from the station ground back to the electrode line and the ordinary ground electrode. This has caused high overvoltages to arise on the neutral busbar because of the impedance of the electrode line, which has necessitated overvoltage protective devices in the form of surge arresters, which have had to be amply dimensioned both from the point of view of voltage and energy. The overvoltage protection devices have therefore become costly. Further, the mechanical circuit breakers have constituted an undesirable complication of an HVDC station. Because of the impedance of the electrode line, overvoltages on the neutral busbar in an HVDC station can also occur at an overcurrent on the electrode line. Also such overvoltages necessitate amply dimensioned surge arresters in known HVDC stations. Similar problems arise if, in case of a fault in one of the poles of a bipolar transmission, it is desired to transfer the direct current from this pole to the electrode line and the ground electrode to be able to continue the operation of the plant as monopolar operation with the faultless pole. The case will be the same if it is desired to transfer the direct current back from the electrode line line to a so-called metallic return, that is, to the line of the faulty pole, for a more permanent monopolar operation. Similar problems also arise if, in a bipolar transmission, during monopolar operation with a metallic return, it is desired to transfer the operating current to the electrode line, for example as a first step in a change to a bipolar operation. The problems have been described above with particular reference to a plant for HVDC power transmission, but similar problems arise generally in direct current circuits when transferring a direct current from one current path to another, whereby in a known manner the circuit impedances, and in particular the circuit inductances, render the transfer difficult and cause overvoltages. The invention aims at developing a switching device for a direct current circuit for transferring a current from one current path to another current path, which device is particularly suited for circuits with current paths comprising large inductances and operating at high voltages and/or with large currents which make the current transfer difficult by causing high overvoltages. Further, the invention aims to provide a switching device of the above-mentioned kind by which, in a simple and efficient manner, a limitation of the overvoltage level in the circuit can be achieved, and which therefore allows the necessary overvoltage protective devices to become considerably simplified and less expensive. Finally, the invention aims to provide a simple switching device of the above-mentioned kind, in which the transfer of the direct current can be made completely without mechanical breakers. To achieve this aim the invention suggests a switching device according to the introductory part of claim 1, which is characterized by the features of the characterizing part of claim 1. Further developments of the invention are characterized by the features of the additional claims. The switching device according to the invention is particularly suited for converter stations in high-voltage direct current power transmission plants. By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in Figure 1 an example of a converter station in a bipolar HVDC transmission, which station is arranged according to the invention, Figures 2-5 different modes of operation of the station shown in Figure 1, Figure 2 the function during transfer of the current from the station ground to the electrode line and also the function for limiting the voltage of the neutral busbar, Figure 3 the function during transfer of the direct current from a faulty pole to the electrode line, Figure 4 the function during transfer of the direct current from the electrode line to the line of the faulty pole for changing to monopolar operation with so-called metallic return, Figure 5 the function in the event that, during monopolar operation with metallic return, it is desired to transfer the operating current to the electrode line, for example as a first step during a change to bipolar operation, Figure 6a schematically a general switching device according to the invention connected between a current source and to different load branches, Figure 6b the same general switching device as in Figure 6a connected via two direct current lines to a single load circuit. Figure 7a and Figure 7b two examples of general devices according to the invention, which are adapted for transfer of a direct current with an arbitrary direction from an arbitrary terminal to one of the other two terminals of a device, Figure 8 an alternative embodiment of the device according to the invention, wherein the number of semiconductor elements has been reduced by using a number of mechanical disconnecting switches, and Figure 9 an example of a control device for a device according to the invention. Figure 1 shows a station ST in an HVDC power transmission plant. The station is a bipolar station, that is, it has two series-connected converters SR1 and SR2, each one being connected to a direct current line DL1 and DL2, respectively, via a smoothing reactor, RE1 and RE2, respectively. The point of connection PO between the two poles is connected to an electrode line EL, which leads to a ground electrode GE, arranged at a distance from the station ST. The inductance and resistance, respectively, of the electrode line are symbolically shown in the figure as the discrete impedances LL and RL, respectively. The direct current in the electrode line is designated I dE . The point of connection PO is located at the neutral busbar NS of the station, which busbar extends between the two converters SR1 and SR2. In case of drop out of one of the two poles of the plant, the plant can be operated in monopolar operation, in which case the direct current of the transmission is conducted in one direction by the direct current line of the faultless pole and in the other direction, either via ground by means of the electrode line EL and the ground electrode GE, or - after switching - via the direct current line of the faulty pole (so called metallic return). The station has two disconnecting switches, F21 and F22, by means of which each pole line can be disconnected from the respective converter, for example for using the pole line as metallic return. The station is provided with a ground connection, the so-called station ground SG, arranged in or near the station. Under certain conditions, for example at overvoltages, or in case of a fault on the electrode line, the neutral busbar NS can be temporarily grounded in the station. The busbar is then connected to the station ground SG by means of a switching member, for example a mechanical electric switching device, or alternatively, as will be described below, an electrical switching member in the form of a thyristor connection. As shown in Figure 1, the station has a number of turn-off thyristor connections, X1, X2, W1, W2, U1, U2, Y1, Y2, Z1, Z2, which are capable of being turned off by means of a control signal. In dependence on the current and voltage conditions, each thyristor connection may consist of one single gate turn-off thyristor or a series connection, parallel connection or series-parallel connection of more than one gate turn-off thyristors. Alternatively, a turn-off thyristor connection may consist of one or mare conventional thyristors which are not capable of being turned off via a control signal and which are provided with external turn-off circuits, or of other semiconductor devices which are controllable between a conducting and a non-conducting states. Certain of the thyristor connections may during a certain interval be supplied with a continuous firing pulse such that they are maintained in conducting state during the interval and have the same function as diodes. Certain of the thyristor connections can be controlled on and off in dependence on a sensed voltage, in particular on the voltage across one of the capacitors of the device, which will be described in more detail below. Further, the device comprises two normally closed disconnecting switches F11 and F12, arranged in the neutral busbar on either side of the connection point PO of the electrode line, as well as a normally open disconnecting switch FO, by means of which the connection points P51 and P52, respectively, between the thyristor connections Y1 and Z1 and the thyristor connections Y2 and Z2, respectively, can be connected. Further, disconnecting switches F31 and F32 are provided for use during monopolar operation with metallic return. The device also comprises two capacitors C11 and C12, arranged between the neutral busbar NS and the station ground SG, as well as two capacitors C21 and C22, each one connected - via a pair of anti-parallel thyristor connections (e.g. W1, U1) - in parallel with two thyristor connections (e.g. Y1, Z1) arranged in the neutral busbar. Each capacitor is provided with a voltage measuring member for measuring the capacitor voltage, that is, for generating a measurement signal corresponding to the capacitor voltage. The measuring members are shown in the figure as voltage dividers UM11, UM12, UM21, UM22. The capacitors may be of any conventional kind which is used for harmonic filtering and overvoltage damping in HVDC plants, possibly oversized taking into consideration their function according to the invention. During undisturbed bipolar operation, the disconnecting switches F11, F12, F21 and F22 are closed and the other switches open. The direct currents I d1 and I d2 of the two poles are equally great, and the current in the electrode line EL is therefore zero. The thyristor connections Y1, Z1, Y2 and Z2 are kept continuously conducting. The direct current therefore flows along the current path DL2, F22, RE2, SR2, Y2, Z2, F12, Fll, Z1, Y1, SR1, RE1, F21, DL1. As mentioned in the introductory part of the description, it is known, in certain fault cases, for example an interruption in the electrode line, to temporarily connect the neutral busbar to the station ground. After termination of the disturbance, the operating direct current has been transferred from the station ground back to the electrode line and the ordinary ground electrode. Figure 2 shows this function, that is, the transfer of the operating direct current of the upper pole in the figure from station ground to the electrode line, and also the function for limiting the voltage of the neutral busbar, for example at an overcurrent in the electrode line. In the figure (and in Figures 3-5), the current paths used during the function are indicated in thick lines. Of the turn-off thyristor connections included in these current paths, those which are kept continuously conducting, that is, which have a diode function, are indicated by filled symbols. The figure only shows the current paths in the upper pole, since the lower pole in the case shown in Figure 2 may be assumed to be put out of operation. A fault, for example in the electrode line during monopolar operation, is assumed to have caused the neutral busbar NS to be connected to station ground by firing of the thyristor connection X1. The current I d1 thus initially flows from the station ground SG through the thyristor connection X1 and further through the converter SR1 to the pole line DL1. After correcting the fault, it is desired to resume operation with the electrode line, and the operating current I d1 is therefore to be transferred from the current path SG, X1 to the current path GE, EL, F11, Z1, Y1. The thyristor connections Y1 and Z1 are assumed during the process described below to be kept continuously conducting and function as diodes. To start the transfer of the current, X1 is turned off. The current then flows in the path SG, C11, Y1 and charges the capacitor with the polarity shown in the figure. The measuring device UM11 continuously delivers a measurement signal to a control device (not shown in the figure) for the thyristor connection X1, which measurement signal is a measure of the capacitor voltage U C11 . The control device may be designed in the manner shown in Figure 9 and delivers a firing signal to X1 when the capacitor voltage reaches an upper limit value. The thyristor connection X1 now takes over the operating current, and the capacitor is discharged. When the capacitor voltage has dropped to a lower limit value, X1 is again turned off, and the procedure continues automatically with X1 operating in pulsed operation. The thyristor connection Y1 operating as a diode prevents the capacitor from being discharged through the thyristor connection X1 upon firing of the latter. During this whole procedure, the voltage of a neutral busbar is negative relative to ground. The negative voltage causes a continuously growing current through the electrode line with the direction indicated in the figure, and a corresponding decay of the current from the station ground. The rate of growth of the electrode line current then primarily depends on the inductance of the electrode line and on the mean value of the voltage of the neutral busbar, that is, on the two voltage limit values mentioned. As the current of the electrode line grows, an increasingly shorter time is needed for discharging the capacitor from the upper to the lower voltage limit value, that is, the conduction intervals for the thyristor connection X1 become shorter and shorter, until the electrode line current has reached the value I d1 . The transfer of the direct current to the electrode line is then completed, and X1 ceases to fire. The direct current I d1 now flows along the current path GE, EL, F11, Z1, Y1, SR1, and so on. The function described with reference to Figure 2 is substantially the same at an overvoltage on the neutral busbar NS caused by the converter. At an overvoltage with a negative polarity relative to ground (caused by SR1), and of such a magnitude that it reaches the above-mentioned upper voltage limit value, the thyristor connection X1 is automatically fired, whereby the overvoltage is limited, and the thyristor is automatically turned off if the overvoltage drops to the above-mentioned lower limit value. A positive overvoltage is taken care of in a corresponding manner by the corresponding members (thyristor connection X2, capacitor C12, measuring member UM12) of the device in the lower half of the figure. Overvoltages caused by external events such as, for example, lightning, can be limited in a simple manner by a conventional surge arrester of, for example, zinc oxide. Figure 3 shows the function of the device when it is used for transferring the direct current from a faulty pole to the electrode line. A fault is assumed to have occurred in the upper pole in the figure, and it is desired that the operating direct current of the pole be transferred to the electrode line for maintaining the operation as a monopolar transmission. During the transfer of the current, the thyristor connections U1, W1, Y2 and Z2 are kept continuously turned on, and they will therefore operate as diodes. The normally continuously conducting Z1 is kept turned off. The disconnecting switch FO is closed, and since Y1 is assumed to be turned on, the operating current now flows along the path SR2, Y2, FO, Y1, SR1, etc. The thyristor connection Y1 is now turned off. The direct current of the lower pole then flows along the path Y2, Z2, F12, F11, C21, W1, SR1, etc. The capacitor C21 is then charged by the current with the polarity of the capacitor voltage U C21 as shown in the figure. This voltage is measured continuously by the measuring device UM21. When the capacitor voltage has reached an upper limit value, Y1 is fired, whereby the current will again flow along the path Y2, FO, Y1, SR1. During this process, the capacitor is discharged through the current path U1, C21, EL. When the capacitor voltage has dropped to a lower limit value, Y1 is again turned off. In a manner analogous to that described with reference to Figure 2, the capacitor will be maintained at a voltage, the mean value of which lies between the two limit values mentioned. This voltage causes a successively increasing current in the electrode line and a successively decreasing current through the upper pole. This process continuous until the direct current of the upper pole has, in principle, dropped to zero and the whole direct current of the lower pole flows through the electrode line. Figure 4 shows the function while assuming that the lower pole has been taken out of operation because of a fault, and that its current has been transferred to the electrode line. The plant operates in monopolar operation. To avoid continuous ground currents during a more permanent monopolar operation, it is desired to transfer the operating current from the electrode line EL to the line DL2 of the faulty lower pole for operation with so-called metallic return. The direct current I d1 of the upper pole initially flows, after disconnection of the converter of a lower pole, from the ground electrode GE through the electrode line EL and the continuously turned-on thyristor connections Z1 and Y1. The transfer of the current is started by closing the disconnecting switch F32 and maintaining the thyristor connection W1 continuously turned on. The transfer is initiated by turning off Y1 and/or Z1. The direct current then commutates to the current path through C21 and W1, and the capacitor is charged with the shown polarity of the capacitor voltage u C21 . The afore turned off thyristor connection(s) Y1 and/or Z1 is/are turned on and thereafter turned off in dependence on the capacitor voltage in the same way as described previously. The capacitor voltage causes a successively decreasing current through the electrode line and a successively increasing current through the pole line DL2, and the transfer is automatically terminated when the whole operating current has been taken over by the pole line DL2. Figure 5 shows the function in the case where during monopolar operation with the upper pole in the figure in operation and with metallic return through the pole line of the lower pole, it is desired to transmit the operating current to the electrode line, for example as a first step in a return to bipolar operation after removal of a fault in the converter of the lower pole. The current initially flows along the path DL2, F32, W1, SR1, etc. During the transfer, Y1 and Z1 are maintained continuously turned on. The transfer of the current is initiated by turning off W1, whereby the current is taken over by the current path C21, Z1, Y1. This results in a discharge of the capacitor C21 with the polarity of the capacitor voltage u C21 shown in the figure. W1 is turned on and off in a manner analogous to that described above, and the capacitor voltage thereby all the time has the polarity shown. This voltage causes a successively increasing current through the electrode line EL and a successively decreasing current through the pole line DL2. After completed transfer of the current, the disconnecting switch F32 can be opened. The device described above is a special case of a more general device according to the invention. This will be described below with reference to Figures 6-8. Figure 6a shows the principle of a switching device SWD according to the invention. It has three outer connections Q1, Q2 and Q3. A direct current flows through the device between two of these connections. The device is intended for simple transfer of the current from one of these connections to the third connection. Direct current circuits often display large inductances, and a transfer in a simple manner is therefore normally not possible without unacceptably high overvoltages arising. In the example shown in Figure 6a, a direct current I d is assumed to flow in through the connection Q1 and out through the connection Q2. The switching device according to the invention is intended to make possible a transfer of the current from the connection Q2 to the connection Q3 without generating excessive overvoltages. The switching device SWD according to the invention is connected in Figure 6a into, for example, a circuit with a current source CS and two load circuits CC1 and CC2. The current source CS may be a current source in the real meaning of the word, that is, having a high internal impedance, such as a static rectifier with a direct current smoothing inductance. Alternatively, it may be any other object or network, with high or low internal impedance, and capable of supplying a direct current. The switching device SWD in Figure 6a is connected with its terminals Q1 to the current source and with its terminals Q2 and Q3 to the load circuits CC1 and CC2. In the embodiment shown in Figure 6a, for the sake of simplicity, the load circuits are shown as being inductive (inductances L1 and L2) and resistive (resistances R1 and R2. Alternatively, the load circuits may have other configurations and consist of other impedance combinations or direct current networks, or comprise further circuit elements. Each load circuit may, for instance, consist of a load object, such as a static converter or a direct current motor. Alternatively, a load circuit may consist of a direct current line, such as the electrode line between an HVDC converter station and its ground electrode, or of a combination of direct current lines and load objects. The switching device SWD has a gate turn-off thyristor or turn-off thyristor connection TY connected between the connections Q1 and Q2 and with its conducting direction coinciding with the current I d flowing through the connection Q1. The current before the transfer is indicated by unbroken arrows and after the transfer by dashed arrows. Further, the device has a diode D connected between the connections Q1 and Q3 and with its conducting direction coinciding with the current direction after the transfer. Further, a capacitor C is connected between the two connections Q2 and Q3, between which the current is to be commutated. The thyristor TY has a control device SD, which can turn the thyristor on and off in dependence on the voltage U C across the capacitor. The voltage is sensed by a measuring device UM which delivers a measurement signal U mC , corresponding to the voltage, to the control device SD. Initially, the thyristor TY is continuously conducting and operates as a diode, for example by the control device delivering a continuous firing signal to the thyristor. The current Id from the source CS flows in through Q1, through TY and out through Q2 and then through the load circuit CC1 (unbroken arrows). For transfer of the current from Q2 to Q3, i.e. from load CC1 to load CC2, the control device is switched to control of the thyristor in dependence on the capacitor voltage Uc. The control device is thereby adapted to turn on the thyristor when the voltage Uc reaches or exceeds a predetermined upper limit value and to turn off the thyristor when the voltage has dropped to or below a predetermined lower limit value. Initially, it is assumed that the capacitor is discharged, its voltage is zero and the control device therefore turns the thyristor off. The current Id then flows through the diode D and the capacitor and charges the latter with the polarity indicated in the figure. The capacitor voltage successively reaches the upper limit value, whereby the control device turns on the thyristor which takes over the current. During this process, the diode D prevents the capacitor from being discharged through the thyristor. The capacitor discharges over the two load branches CC1 and CC2, thereby building up an increasing current out through the connection Q3 and the load branch CC2 and at the same time a decreasing current out through the connection Q2 and through the load CC1. In this way, the capacitor is discharged, that is, its voltage drops, and when it has reached the lower limit value, the thyristor is again turned off and the capacitor is charged. In this way, by repeated firings and extinctions of the thyristor, the capacitor voltage will be maintained between the two limit values mentioned. The current out through Q2 decreases, and the current out through Q3 increases, at a rate determined by the capacitor voltage and by the circuit inductances. Finally, the current through Q2 and hence the charging current of the capacitor drop to such an extent that the thyristor is no longer fired, and the transfer of the current from Q2 to Q3 is then completed. Thus, the current Id initially flowing in the current path through CS and CC1 (unbroken arrows) is switched over to the current path through CS and CC2 by means of the device SWD, and this in a rapid and controlled manner without risk of overvoltages. The device is intended for transfer of a current with the polarity shown in Figure 6a. For the opposite polarity, the conducting directions of the thyristor and the diode shall be the opposite. The choice of the upper limit value for the capacitor voltage must be made taking into consideration the voltage withstand capability of the circuit and the components as well as the desired rate of transfer. The choice of the lower limit value must be made taking into consideration, inter alia, the desired pulse frequency of the thyristor, and also the desired rate of transfer. Figure 6b shows an alternative use of the switching device SWD of Figure 6a. The inner structure of the device SWD in Figure 6b is identical with that of the device SWD in Figure 6a and has been, for sake of simplicity, omitted in Figure 6b. In this case the switching device SWD is used for transfer of a direct load current from a first direct current line DCL1 to a second direct current line DCL2, both lines leading to a single load circuit CC, which may be any arbitrary kind of direct current load circuit, object or network. The two lines have inductances L1 and L2, respectively, and resistances R1 and R2, respectively. In the same manner as described above, the switching device SWD controls the transfer of the direct current I d from line DCL1 to line DCL2 in a controlled manner. Figure 7a shows a device according to the invention, which is designed so as to make possible transfer of current with an arbitrary polarity from any one to any one of the three outer connections Q1 - Q3 of the device. The device has a mid-point MP. To each connection there are connected a first gate turn-off thyristor A1, A2, A3 with its conducting direction towards the mid-point, and a second thyristor B1, B2, B3 with its conducting direction away from the mid-point. Between each pair of connections there is connected a capacitor C12, C23, C31 and in series with each capacitor there is connected a switching member SW12, SW23, SW31. The switching members may consist of mechanical electric switching devices, such as disconnecting switches, or of bidirectionally conducting semiconductor members. The function of the device according to Figure 7a completely follows the principle described with reference to Figure 6a. The thyristor which is connected to that connection which is not to be concerned by the transfer, and which has a conducting direction corresponding to the current in that connection, is continuously supplied with a control pulse and operates as a diode. The thyristor which is connected to that connection which is to carry current after the transfer, and which has a polarity corresponding to the conducting direction of that current, is also maintained continuously conducting thus having a diode function. That capacitor which lies between those two connections between which the current is to be transferred, is switched in by closing the electric switching device (SW23) thereof. The electric switching devices of the other two capacitors are to be open. The thyristor which is connected to that connection which is not to carry current after the transfer, and which has a conducting direction corresponding to that of the current prior to the transfer, is turned on and off in dependence on the voltage of the switched-in capacitor in the manner described with reference to Figure 6a. For Figure 7a the same assumptions are made as in connection with Figure 6a. This means that the thyristors A1 and B3 in Figure 7a are controlled to function like diodes, that the capacitor C23 is connected by closing the electric switching device SW23, that the electric switching devices SW31 and SW12 are open, that the thyristor B2 is controlled in dependence on the voltage across the capacitor C23, and that the thyristors B1, A2, A3 are continuously turned off. The current paths used for this operating case are shown in thick lines in the same way as in Figures 1-5, and the currents prior to the transfer are shown by unbroken arrows and the currents after the transfer by dashed arrows. With these assumptions the corresponding function as described for Figure 6a can be obtained with the device in Figure 7a in an analogous manner for an arbitrary combination of connections and for an arbitrary polarity of the current. The device shown in Figure 7a has a "star connection" of three thyristor branches connected to its three connections Q1 - Q3, each branch comprising two antiparallel-connected gate turn-off thyristors (or series connections of such thyristors). Alternatively, the three thyristor branches may be "delta-connected", whereby a branch with two antiparallel-connected thyristors is connected between each pair of the connections of the device. Such a device according to the invention is shown in Figure 7b. It has the same components as the device according to Figure 7a, and the designations are analogous to those in Figure 7a. The device in Figure 7b is shown for the same assumed operating case as in Figures 6a and 7a. The switching devices according to Figures 7a and 7b can be connected with their three connection points Q1 - Q3 in the same manner to a current source CS and load circuits CC2, CC1, DCL1, DCL2 and CC as shown for the switching device SWD in Figures 6a and 6b. In, for instance, the embodiment described above in connection with Figure 6a, the capacitor branch C is connected directly to those terminals of the two semiconductor connections D and TY, which face away from the branch point Q0, namely the cathode terminals. The first and the second current path may, of course, within the scope of the invention, alternatively comprise components or elements connected into the current paths, between the semiconductor elements and the connection points of the capacitor branch. Such components or elements may, for instance, be current measuring units or impedances. Also, the capacitor branches of the embodiments described above with reference to Figures 1 to 5 comprise capacitors only but may within the scope of the invention comprise additional components or elements. Figure 8 shows an alternative embodiment of a device according to the invention, in which only one turn-off thyristor branch TY, one diode branch D and one capacitor branch C are required. Since the device only has one single capacitor branch, only one single voltage measuring member UM is required. The three branches mentioned are connected in a triangle. Each one of the three corners P1, P2, P3 of the triangle can be connected with the aid of a set SW of electric switching devices, for example disconnecting switches, to any one of the three connections Q1, Q2, Q3 of the device. For each combination of connections, the device can be brought to assume the configuration described above with reference to Figures 6 and 7 by operation of the electric switching devices. For example, for the case described in Figure 6a with transfer of a current from Q2 to Q3, P1 is connected to Q2, P2 to Q1 and P3 to Q3. The connection members are shown with these positions in Figure 8. The device shown in Figure 8 can only be used in the event that the current flows into the device through that connection which is not influenced by the transfer. If desired, the device can be supplemented in a simple manner so as to make it useful for an arbitrary polarity of the current. This can be done, for example, by providing the thyristor branch and the diode branch with mechanical pole changers. Alternatively, the thyristor branch can be connected in anti-parallel with an additional thyristor branch, in which case electric switching devices are arranged in the two branches for connection of that branch which, on each occasion, has the desired conducting direction. The same is done with the diode branch. Figure 9 schematically shows an example of the configuration of a control device for a device according to the invention, for example the control device SD in Figure 6a. The measurement signal U mC from the measuring member UM is supplied to two level-sensing members V11 and V12, for example so-called level flip-flops. The flip-flop V11 delivers a signal tp when the measurement signal reaches a predetermined reference level u t . The signal is supplied to a differentiating input of an OR circuit OG11. The output signal st of the OR circuit is supplied to an amplifier F11, the output signal st' of which is supplied to the thyristor connection TY in the form of a firing pulse, which results in firing of the thyristor or the thyristors in the thyristor connection. The flip-flop V12 delivers a signal sp when the measurement signal is below a predetermined reference level, u s , which is lower than the reference level u t . The signal from the flip-flop is supplied to a differentiating input of an OR circuit OG12. The output signal ss of the OR circuit is supplied to an amplifier F12. The output signal ss' of the amplifier is supplied to the thyristor connection TY in the form of a turn-off pulse for turning off the thyristor or the thyristors in the connection. In an HVDC transmission with a long electrode line, the reference levels u s and u t , respectively, may be, for example, 65 kV and 80 kV, respectively, and in a transmission with a short electrode line, the reference levels may be of the order of magnitude of 10 kV. The OR circuit OG11 of the control device is provided with a further input adapted, in those cases where a diode function is to be imparted to the thyristor TY, to be supplied with a continuous signal ct, resulting in the thyristor being supplied with a continuous firing signal st' and being continuously maintained conducting. To ensure that the thyristor is non-conducting in those operating cases where it is to assume this state, the OR circuit OG12 is provided in a corresponding manner with a further input adapted to be supplied, in these operating cases, with a continuous signal cs'. This results in a firing signal ss' being supplied to the thyristor and ensures that it changes into, or remains in, the non-conducting state. The embodiment of a device according to the invention described above is only one of many feasible examples. Thus, the invention is described above in an application to a bipolar HVDC transmission, but it can also be applied to a monopolar transmission with the corresponding advantages. Also, within the scope of the invention, the components of the device and the interconnection of these components can be designed in a manner different from that described above. Instead of the turn-off thyristor connections described above, other suitable connection members may be used, for example controllable semiconductor members of a different type. The above description shows how the function of the device is controlled by directly sensing the voltage of a capacitor. Alternatively, the capacitor voltage can be sensed in a more indirect manner. For example, in the above-described application of the device for protection of a neutral busbar in an HVDC station against overvoltages, the current through a surge arrester arranged between the neutral busbar and ground can be sensed and used for controlling the turn-off thyristor connection in a manner corresponding to that described above. To increase the current-handling capacity of the turn-off thyristor connection, two or more thyristors or thyristor branches may possibly be connected in parallel. These can then suitably be adapted to conduct alternately with a suitable periodicity. For the purpose of ensuring that the current-handling capacity of a turn-off thyristor connection is not exceeded, a current measuring device may possibly be arranged to measure the current through the thyristor connection. If this current reaches a predetermined upper limit value, a conventional thyristor (or thyristor connection) connected in parallel with the turn-off thyristor connection is fired, and/or a normally open mechanical electric switching device, also connected in parallel with the turn-off thyristor connection, is closed. To reduce voltage drops and losses, continuously conducting semiconductor members may, if desired, be by-passed by means of mechanical electric switching devices.

Claims: Switching device in a high voltage direct current power transmission system, with a first current path (Q1-Q2) and a second current path (Q1-Q3), said first and second current paths being interconnected at a branch point (Q0), characterized a first semiconductor connection (TY, B2, A12) which is connected into the first current path only and which is controllable between conducting and non-conducting states and vice versa, a second semiconductor connection (D, B3, B31) with a diode function which is connected into the second current path only, a capacitor branch comprising a capacitor (C, C23), and which is connected between a first connection point on said first current path and a second connection point on said second current path, said first connection point and said branch point (QO) being located on opposite sides of said first semiconductor connection (TY, B2, A12), said second connection point and said branch point (QO) being located on opposite sides of said second semiconductor connection (D, B3, B31), and control members (SD) adapted, in dependence on the voltage (U C ) across the capacitor, to control the first semiconductor connection (TY, B2, A12) between a non-conducting and a conducting state. Device according to claim 1, characterized in that the control members (SD) are adapted to be supplied with a quantity (Um C ) corresponding to the capacitor voltage (U C ) to control the first semiconductor connection (TY, B2, A12) to a conducting state if said quantity reaches or exceeds a predetermined upper limit value (u t ), and to control the first semiconductor connection to a non-conducting state if said quantity drops to or becomes lower than a predetermined lower limit value (u s ). Device according to claim 1 or 2, characterized in that at least the first semiconductor connection comprises a thyristor (TY). Device according to claim 3, characterized in that at least the first semiconductor connection comprises a thyristor (TY) capable of being turned off by a control signal (ss'). Device according to any of the preceding claims, characterized in that the second semiconductor connection comprises a thyristor (B3, B31), and that the control members (SD) are adapted to supply to the thyristor a continuous control signal (st') to achieve a diode function of the thyristor. Device according to any of the preceding claims for transferring a direct current (Id) from the first current path (Q1-Q2) to the second current path (Q1-Q3), characterized in that the control members (SD) are adapted prior to the transfer, to maintain the first semiconductor connection (TY, B2, A12) continuously conducting, for achieving the transfer, to control this semiconductor connection to a non-conducting state, and thereafter, in dependence on the voltage (U C ) across the capacitor (C, C23), to control the semiconductor connection between conducting and non-conducting states. Device according to any of the preceding claims for limiting the voltage between, on the one hand, a point (P51) in the second current path and, on the other hand, a point (SG) with a given potential, characterized in that the capacitor branch (C11) is arranged between the point (P51) in the second current path and the point (SG) with a given potential, the first semiconductor connection (X1) is arranged between the point with a given potential (SG) and the branch point (P41), the second semiconductor connection (Y1) is arranged between the branch point (P41) and said point (P51) in the second current path, and the control members (SD) are adapted to turn on the first semiconductor connection if the voltage difference between the point in the second current path and the point with a given potential exceeds a predetermined limit value (u t ). Device according to any of the preceding claims, characterized in that it comprises three connections (Q1, Q2, Q3) and three semiconductor branches (A1, B1; A2, B2; A3, B3) connected in a star connection between said connections and a common point (MP), each semiconductor branch being controllable between conducting and non-conducting states and vice versa. Device according to any of claims 1-5, characterized in that it comprises three connections (Q1, Q2, Q3) and three semiconductor branches (A12, B12; A23, B23; A31, B31) connected in a delta connection between said connections, each semiconductor branch being controllable between conducting and non-conducting states and vice versa. Device according to claim 6 or 7, characterized in that each semiconductor branch is adapted to be controllable to a conducting state in both current directions. Device according to any of claims 1-5, characterized in that it comprises three connections (Q1, Q2, Q3), three connection points (P1, P2, P3), one capacitor branch (C), two semiconductor branches (TY, D), and switching members (SW) for switchable connection of the connection points to the connections, each pair of connection points is connected by means of one of said branches, one of the semiconductor branches comprises said first semiconductor connection (TY), and the second semiconductor branch comprises said second semiconductor connection (D). Device according to any of claims 1-7 in a converter station (ST) in a plant for power transmission by means of high-voltage direct current, which station has a neutral busbar (NS), a converter (SR1) connected between the neutral busbar and a pole line (DL1), and a station ground (SG) connectible to the neutral busbar, characterized in that the first semiconductor connection (X1) is connected between station ground and a first point (P41) on the neutral busbar, the capacitor branch (C11) is connected between station ground and a second point (P51) on the neutral busbar, and the second semiconductor connection (Y1) is connected between said first (P41) and second (P51) points on the neutral busbar. Device according to claim 12 in a converter station (ST) in a plant for a bipolar power transmission, which station comprises an electrode line (EL), connected to a neutral point (P0) on the neutral busbar, said electrode line (EL) being connected to a ground electrode (GE) arranged spaced from the station, a first converter (SR1) connected between the neutral busbar and a first pole line (DL1), a second converter (SR2) connected between the neutral busbar and a second pole line (DL2), and in which the neutral busbar has a first part located between the neutral point and the first converter and a second part located between the second converter and the neutral point, characterized in that each one of the two parts of the neutral busbar has a capacitor branch (C11, C12) and a first semiconductor connection (X1, X2) connected to the station ground and a second semiconductor device (Y1, Y2) connected into the neutral busbar. Device according to any of claims 1-7, 12 or 13 in a converter station (ST) in a plant for bipolar power transmission, which station comprises a neutral busbar (NS), an electrode line (EL) connectible to a neutral point (P0) on the neutral busbar, a first converter (SR1) connected between the neutral busbar and a first pole line (DL1), a second converter (SR2) connected between the neutral busbar and a second pole line (DL2), and in which the neutral busbar has a first part located between the neutral point and the first converter and a second part located between the second converter and the neutral point, characterized in that each one of the two parts of the neutral busbar has two series-connected semiconductor members (Y1, Z1; Y2, Z2) controllable between conducting and non-conducting states and vice versa, a series connection of a capacitor (C21, C22) and a bidirectionally controllable semiconductor member (U1, W1; U2, W2), said series connection being connected in parallel with the series-connected semiconductor members, and control members adapted to control at least one of said series-connected semiconductor members (Y1, Z1; Y2, Z2) and bidirectionally controllable semiconductor members (U1, W1; U2, W2) in dependence on the capacitor voltage (U C21 , U C22 ). Device according to claim 14, characterized in that it has connection members (F0) for connection of the connection point (P51) of the series-connected semiconductor members in one part of the neutral busbar to the corresponding point (P52) in the other part of the neutral busbar. Device according to claim 14 or 15, characterized in that it has connection members (e.g. F32) for connection of the pole line (DL2), which is connected to one part of the neutral busbar, to the connection point (P61) between the capacitor (C21) associated with the other part of the neutral busbar and the bidirectionally controllable semiconductor member (U1, W1) associated with the same part of the neutral busbar. Device according to any of claims 14-16, characterized in that at least certain of said series-connected semiconductor members and bidirectionally controllable semiconductor members consist of thyristors. Device according to claim 17, characterized in that at least certain of the thyristors consist of thyristors (TY) which are capable of being turned off by a control signal (ss'). Device according to claim 17 or 18, characterized in that it comprises semiconductor members (SD) adapted to supply at least some of the thyristors with a continuous control signal (st') to achieve diode function of the thyristors.

Inventors: BJOERKLUND PER-ERIK, BJOERKLUND, PER-ERIK, Björklund, Per-Erik
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1994
Main classification: H02J3
IPC codes: H02J3, H02J1